Cathodic protection is defined as either (1) the reduction of corrosion rate by shifting the corrosion potential of the electrode toward a less oxidizing potential by applying an external electromotive force or (2) the partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or impressed current. Cathodic protection is well established method by which a metal can be protected from corrosion. It is used anywhere the structure and anode are embedded in a common electrolyte, such as, on pipelines, tanks, piers, pilings, bridge decks and substructures.
Cathodic protection is accomplished by causing a flow of direct current (DC) between another electrode (called the anode) and the structure (called the cathode). The DC causes the surface of the structure to become polarized, thus stopping or reducing corrosion.
In an impressed current cathodic protection system, a rectifier normally converts AC to DC and supplies the current; and the anode is a relatively inert material that can transfer the current to the soil or water, which is the normal electrolyte. In a galvanic system, the anode is an electrochemically active metal compared to the cathode, and the current is a natural occurrence of connecting the anode and cathode together.
The chemical reaction at the anode is metal oxidation, with oxygen, or chlorine evolution. Both ions and electrons are formed at the anode. Ions generated by the reaction flow to the cathode via the electrolyte. The electrons flow to the cathode via a connection. Reduction occurs at the cathode, which consumes the electrons. Three common cathodic reactions in cathodic protection are as follows: EQU O.sub.2 +2H.sub.2 O+4e.fwdarw.4OH.sup.- EQU O.sub.2 +4H.sup.+ +4e.fwdarw.2H.sub.2 O EQU 2H.sup.+ +2e.fwdarw.H.sub.2
In all cases, both electron and ionic currents are involved in cathodic protection. That is, both a continuous common electrolyte and a metallic connection between the anode and cathode are required. The metallic connection is provided through the rectifier. The soil, water, or other conductive medium provides the common electrolyte.
In cathodic protection practice, anodes may be installed in the ground or water where the anodic reactions take place. Anodes can be installed in several different configurations depending on the requirements; these include (1) a single point using shallow anodes, (2) evenly spaced along the structure (distribution system), or (3) in a deep well. Common anode materials include pure metals, alloyed metals, platinum coated valve metals, valve metals having electrochemically active coatings, and certain ceramic materials.
If a separation or void in the electrolyte occurs between the anode and cathode, the current will stop, and cathodic protection will fail. Conditions where the electrolyte becomes discontinuous between anode and cathode can and do occur in practice. Examples of situations where this could occur include: coarse backfill materials that are too coarse to permit intimate or uniform contact with the structure surface (e.g., gravel, rocks); where fine backfill is washed away from the environment surrounding the structure from groundwater flow leaving voids against the structure containing only air; and in exposed situations where fine backfill is not possible or practical to place.
The application of anode material embedded in concrete for the cathodic protection of steel reinforcement in concrete bridge decks and substructures, piers, and buildings is common. The use of an anode embedded in an acidresistant cement liner containing an anode for the cathodic protection of stainless steel flue gas ducts is disclosed in U.S. Pat. No. 5,290,407. This patent applies to the protection of stainless steel flue gas ducts that are exposed to an environment consisting of a thin-film acidic condensate in an otherwise atmospheric environment. There is a significant problem in providing proper cathodic protection with respect to buried steel structures such as pipelines, where the natural environment consists of rocks or other materials that present voids or prevent full contact between the structure and environment.